高放废物处置库北山预选区区域地下水流模拟及岩体渗透特征研究

【作者】 王海龙；

【导师】 郭永海； 王驹；

【作者基本信息】 核工业北京地质研究院 ， 矿产普查与勘探， 2014， 博士

【摘要】 高放废物地质处置的目的就是将高放废物与人类环境有效隔离。高放废物处置库的围岩通常为低渗透的岩体，其安全性在很大程度上取决于围岩对核废物的屏障功能和作为核素迁移载体的裂隙水运动特征。因此，开展处置库场址水文地质评价工作尤为重要。本文依托“甘肃北山预选区选址和场址评价及地质处置技术研究”项目，开展北山地区区域水文地质评价研究。研究首先通过建立大区域地下水流数值模型，模拟区域地下水循环模式，宏观上分析和认识地下水迁移路径，为高放废物处置库选址评价提供区域水文地质依据，并分析潜在的处置库地下水排泄区域；其次，在预选场址尺度上，开展北山预选区重点岩体渗透性空间分布规律研究，通过岩体离散裂隙网络模型的建立和模拟计算，分析围岩裂隙渗透特征，建立预选场址岩体渗透性能评价方法，为处置库选址和场址评价提供数据、方法和理论支持。本文通过对研究区地下水均衡要素、流场及水文地质参数分布的分析和描述，建立了研究区水文地质概念模型，利用GMS软件构建了地下水稳定流数值模拟模型，并利用观测孔水位数据进行模型参数识别。通过模拟计算，对研究区地下水系统水均衡及水质点运移进行了模拟分析。研究表明，1）研究区地下水系统多年平均总补给量为12042.19×104m3/a，总排泄量为12128.5×104m3/a，均衡差为-86.31×104m3/a。地下水主要补给来源是大气降水入渗和沟谷洪流入渗，其值为1418.19×104m3/a。地下水的主要排泄方式为蒸发，蒸发量为9589.17×104m3/a，其次为向相邻含水层的侧向排泄，其中，向黑河下游平原区的侧向排泄量为1068.74×104m3/a，向河西走廊地区的侧向排泄量为1178.21×104m3/a；2）北山预选区区域地下水的主流向是自西向东流动，排泄区为黑河下游平原及河西走廊地区；3）地下水质点轨迹模拟结果表明，从马鬃山地区到河西走廊地区的时间大约需要7.2~16.3万年，而运移到黑河下游平原最快约19.6万年，最慢需要68万年；模型模拟结果表明研究区地下水循环路径长、循环速度慢，且与外界水交换量少，符合高放废物处置库选址的基本条件。本研究以野外裂隙实测数据为基础，统计分析了BS16钻孔围岩芨芨槽岩体的渗透特征。研究结果表明，1）BS16钻孔围岩渗透系数变化范围为10-7m/s~10-14m/s，岩体渗透性在垂向上呈波动分布，空间分布规律不明显；2）BS16钻孔围岩的主渗透方向优势走向为35°、298°和70°，渗透主值各向异性比率C1值主要分布区间为1.3~1.8，平均值为1.46；C2值主要分布在3左右，占总数概率25%。在分析BS16钻孔围岩裂隙发育特征基础上，本文利用Fracman软件建立了BS16钻孔围岩芨芨槽岩体的三维裂隙网络模型，分析了岩体裂隙连通性。研究表明，1）BS16钻孔围岩裂隙按产状分为5组，裂隙优势产状分别为239∠74°、303∠66°、56∠63°、136∠72°和97∠12°。五组裂隙的倾向、倾角基本上服从正态函数曲线分布（第五组裂隙倾向除外），而裂隙间距与迹长主要服从负指数函数曲线分布型式；裂隙在垂向上具有明显分层性，按裂隙密度将围岩在垂向上分为6层，裂隙产状服从正态函数分布；2）BS16钻孔围岩离散裂隙网络模型模拟分析表明，水力连通裂隙集中分布在260m~480m深度范围内，且优势走向为320°，70°和100°；500m深度以下围岩裂隙水力连通性差，是适宜的高放废物处置库建造深度。更多还原

【Abstract】 Deep geological disposal of high-level radioactive waste is considered to bemore stable and safer way to isolate high-level radioactive waste from humanenvironment. Normally, the surrounding rock is expected to be low-permeable matrix.The safety of geological disposal depends on the screen effect of surrounding rockand fluid flow with nuclear waste transport through complex fractured rocks.Therefore, it is critical to conduct hydrogeological assessment at potential disposalsites of interest. Based on the project of “Technical study on the selection andevaluation of potential sites and their relevant geological disposal issues in GansuBeishan region”, this study, at the scale of potential selection site, further made effortson characterizing groundwater circulation, developing a regional numerical model,analyzing controlling factors on groundwater circulation, macroscopically capturinggroundwater flow path, and providing hydrogeological evidences for high-levelradioactive disposal at Beishan region. Additionally, at the scale of potential selectionsite, we conducted researches on capturing spatial distribution of hydraulicconductivity for tentative rocks of interest, developing discrete fracture networkmodeling to simulate hydraulic properties of surrounding rocks, building up a methodto evaluate permeability of matrix, all of which were aimed to provide statistical andtheoretical evidences for selecting and evaluating potential sites.The study developed a hydrogeological conceptual model based on the analyzingand mapping corresponding hydrogeological aspects, including groundwater budget,flow net, and relevant spatial distribution of hydraulic properties. Furthermore, wedeveloped stead-state groundwater numerical model through GMS and used observeddata to calibrate the numerical model. According to the numerical results, we analyzedwater budget for the studied area, and used particle tracking method to simulatenuclear waste movement travelling through the fractured reservoir. Results from thisstudy show that:(1) The average annual recharge to studied groundwater system isaround12042.19×104m3/a, while the average annual discharge is12128.50×104m3/a, with deficiency amounting to-86.31×104m3/a. The recharge magnitude to thegroundwater system is mainly composed of infiltration from precipitation and leakagefrom flood in valleys, which is1418.19×104m3/a. On the other hand, groundwater ismainly discharged in terms of evapotranspiration, which is around9589.17×104m3/a,followed by the lateral discharge to adjacent groundwater system, among which with1068.74×104m3/a is discharged to the downstream of Hehei plain area, whereas with1178.21×104m3/a is discharged to Hexi Corridor region;(2) Groundwater flows bylarge from west to east, where the studied discharge areas are the downstream ofHehei plain and the Hexi Corridor region;(3) According to the particle trackingmethod, it takes around7.2~16.3×106years for particle traveling from theMazongshan area to the Hexi Corridor region, while the shortest and longestresidence time for particle traveling from the Mazongshan area to the downstream ofHehei plain are19.6×106and68×106years. As expected, it takes very long path forparticle circulating through the studied groundwater system and thus results in longresidence time. Moreover, little water mixing and exchanging during circulationprocess. Therefore, Beishan area is considered to be a favorable site for the purpose ofhigh-level radioactive disposal.Based on the data from in-site fracture measurements, this study statisticallyanalyzed hydraulic properties of surrounding rocks around drilling well BS16. Theresults show that:(1) Hydraulic conductivity of surrounding rocks about BS16variesfrom10-7m/s~10-14m/s, and shows a fluctuating distribution in the vertical directionwithout a determined spatial pattern;(2) The principle directions of hydraulicconductivity for surrounding matrix are35°,298°and70°, respectively. Theanisotropic ratio C1ranging between1.3~1.8with average value to be1.46; theaverage anisotropic ratio C2is around3, taking up25%of ensemble samples.Upon analyzing the characteristics of fracture development around BS16, thisstudy developed discrete fracture network through Fracman software, and furtheranalyzed the connectivity of fractures. The results show that:(1) The preferentialattitudes of fractures can be categorized into five groups, which are239∠74°, 303∠66°,56∠63°,136∠72°and97∠12°, respectively. The inclinations anddip angles of those groups normally follow normal Gaussian distribution, except forthe inclination of the fifth group. Additionally, the fracture spacing and length follownegative exponential function. Note that there is a clear vertical stratification forfractured rocks, which can be categorized into six layers according to the fracturedensity distribution. Fracture attitude follows normal Gaussian distribution.(2)According to the results from discrete fracture network modeling at BS16, hydraulicconnection for fracture network is concentrated on the depth between260m~480m,with preferential fracture orientations amounting to320°,70°, and100°. Below thedepth about500m, there is poor hydraulic connection for fracture network, and thus itis a favorable depth for geological disposal of high-level radioactive waste.更多还原